1. Field of the Invention
The present invention relates to a distance measuring equipment for measuring a distance up to a target to be measured in distance in such a manner that light is projected and light reflected on the target and returned is detected, and a camera incorporated thereinto such a distance measuring equipment.
2. Description of the Related Art
Hitherto, there is widely spread a camera incorporating thereinto an automatic focusing (AF) device, which is generally referred to as an active type. Such an active type of automatic focusing device adopts a system for measuring a distance up to a camera subject in such a manner that an AF light projection unit and an AF photo detection unit are disposed at intervals of a predetermined base length, light is projected from the AF light projection unit to the camera subject, and light reflected on the camera subject is detected by the AF photo detection unit.
As a photo detector element for focusing, for example, a PSD element, is used. As an arithmetic device for performing an arithmetic processing in accordance with photo-current from the PSD element, for example, there is used an AF-dedicated photo detector IC for performing a ratio arithmetic processing.
To perform a photography using such a camera, first, a shutter button is subjected to a half-push operation. Then, light for focusing is projected from the AF light projection unit ahead of the camera, and light for focusing, which is reflected on the camera subject and returned, is detected on a detection surface of the PSD element so that a photo-current is generated. The photo-current thus generated is separated into two photo-currents (a photo-current at the short distance side and a photo-current at the long distance side) in accordance with the respective distances between a projection position on the detection surface of the PSD element and both ends of the detection surface. And signals representative of those two photo-currents are fed to the AF-dedicated photo detector IC. In the AF-dedicated photo detector IC, a ratio arithmetic processing is performed in accordance with the signals representative of those two photo-currents. A distance up to the camera subject is determined in accordance with a result of the ratio arithmetic processing, and an image taking lens is moved to the distance thus determined. In this manner, a focusing is performed, and then the shutter button is subjected to a full-push operation to perform a photography.
Generally, as a distance up to the camera subject is far, a photo-current from the PSD element is smaller. Thus, S/N ratio of a signal representative of a photo-current, particularly a signal of a long distance side, is lowered, and it is difficult to exactly determine the distance up to the camera subject by the AF-dedicated photo detector IC.
In view of the foregoing, Japanese Patent Application Laid Open Gazette Hei.4-48208 discloses a technology of enhancing accuracy in focusing for both the short distance and the long distance in such a manner that in case of the short distance for the camera subject, values wherein ratio arithmetic values of the photo-currents outputted from the PSD element are integrated by a predetermined number of times, are given as focusing data, and in case of the long distance for the camera subject, values wherein addition values (light quantity) of the photo-currents outputted from the PSD element are integrated by a predetermined number of times, are given as focusing data.
Further, in view of the foregoing, Japanese Patent Application Laid Open Gazette Hei.7-260475 discloses a technology of enhancing accuracy in focusing for both the short distance and the long distance in such a manner that control data, which are different for each camera, are written beforehand, and the optimum short distance side signal and long distance side signal are determined in such a way that the control data are utilized, when a focusing program is effected, to equally operate first and second amplifiers connected to a pair of output terminals of a PSD element, so that that the short distance side signal and the long distance side signal are fed to ports of a microcomputer for controlling a camera to perform an arithmetic operation.
However, according to the technology proposed in Japanese Patent Application Laid Open Gazette Hei.4-48208, there are individually needed a circuit for computing addition values and a circuit for computing ratio arithmetic values, and thus a circuit scale of a photo detector is enlarged. This causes the cost of the equipment or the device to be increased.
According to the technology proposed in Japanese Patent Application Laid Open Gazette Hei.7-260475, there is associated with such a problem that the number of input ports of the microcomputer is increased.
In view of the foregoing, it is considered that an AF-dedicated photo detector IC, which is capable of performing both the ratio arithmetic processing and the operating processing based on light quantity, is developed to adopt the same. In this case, however, it takes much cost for a development of the new IC per se. And in addition, the development of the new IC cannot bring about a merit in low cost caused by a mass production of the AF-dedicated photo detector IC performing only the ratio arithmetic processing. Thus, also in this case, there is associated with such a problem that the cost is increased.
This problem occurs also in the event that an SPD element having a divided detection surface is used as a photo detector for focusing, not restricted to a case where a PSD element is used as the photo detector for focusing.
In view of the foregoing, it is an object of the present invention to provide a distance measuring equipment capable of performing focusing with greater accuracy, suppressing the cost up, and a camera incorporating thereinto such a distance measuring equipment.
To achieve the above-mentioned object, the present invention provides a distance measuring equipment for measuring a distance up to a target to be measured in distance in such a manner that light is projected and light reflected on the target and returned is detected, said distance measuring equipment comprising:
a semiconductor photo detector element having a detection surface generating a photo-current upon receipt of irradiation of light;
a ratio arithmetic circuit, having two input terminals each for receiving associated one of current signals, for performing a ratio arithmetic operation in accordance with the current signals fed from the two input terminals;
a current source for generating fixed current signal; and
a first signal transmission path wherein the photo-current obtained through irradiation of light onto the detection surface of said semiconductor photo detector is lead to one input terminal of the two input terminals of said ratio arithmetic circuit and the fixed current signal from said current source to another input terminal of the two input terminals of said ratio arithmetic circuit.
According to the distance measuring equipment of the present invention, the photo-current generated on the detection surface of the semiconductor photo detector is fed to one input terminal of the two input terminals of the ratio arithmetic circuit, and the fixed current signal is fed to another input terminal of the two input terminals of said ratio arithmetic circuit, so that a ratio arithmetic processing is performed. This feature makes it possible to perform an arithmetic operation based on light quantity through adopting an algorithm referred to as a ratio arithmetic operation as it is. That is, it is possible to perform an arithmetic operation based on light quantity through adopting the conventional ratio arithmetic circuit as it is.
In the distance measuring equipment according to the present invention as mentioned above, it is preferable that said current source generates fixed current signal of an optional current value.
The use of such a current source makes it possible to control the focusing characteristic representative of a result of a ratio arithmetic operation in the ratio arithmetic circuit, since the value of the fixed current signal to be fed to the ratio arithmetic circuit can be selected.
In the distance measuring equipment according to the present invention as mentioned above, it is acceptable that said semiconductor photo detector element comprises a plurality of signal electrodes for outputting on a share basis the photo-current generated on said detection surface, and
said first signal transmission path leads photo-currents outputted on a share basis from said plurality of signal electrodes to said one input terminal.
In this case, it is acceptable that said semiconductor photo detector element comprises said detection surface, and a pair of signal electrodes, disposed at both ends of said detection surface, adapted for outputting on a share basis the photo-current generated by irradiation of light onto said detection surface in accordance with respective distances between an irradiation position on said detection surface and both ends of said detection surface.
Alternatively, in the distance measuring equipment according to the present invention as mentioned above, it is acceptable that said semiconductor photo detector element comprises said detection surface, a pair of signal electrodes, disposed at both ends of said detection surface, adapted for outputting on a share basis the photo-current generated by irradiation of light onto said detection surface, and a first switch, disposed at a position adjacent to one signal electrode which is one of said pair of signal electrodes, for selectively offering a first state which permits the photo-current generated on said detection surface to conduct into said one signal electrode, and a second state which inhibits the photo-current generated on said detection surface from conducting into said one signal electrode, in accordance with a control signal, and
said first signal transmission path leads, when said first switch offers said second state, the photo-current outputted from another signal electrode different from said one signal electrode of said pair of signal electrodes to said one input terminal.
In this case, it is preferable that in said semiconductor photo detector element, when said first switch is in the first state, said pair of signal electrodes output on a share basis the photo-currents generated by irradiation of light onto said detection surface in accordance with respective distances between an irradiation position of the light on said detection surface and both the ends of the detection surface.
Alternatively, in the distance measuring equipment according to the present invention as mentioned above, it is acceptable that said semiconductor photo detector element comprises said detection surface, signal electrodes, connected to associated divided areas of said detection surface, respectively, adapted for outputting the photo-current generated by irradiation of light onto said detection surface, and a second switch, disposed at a boundary between divided areas of said detection surface, for selectively offering a first state which permits the photo-current generated on said detection surface to conduct through the associated boundary, and a second state which inhibits the photo-current generated on said detection surface from conducting through the associated boundary, in accordance with a control signal, and
said first signal transmission path leads, when said second switch offers said first state, the photo-current outputted from the signal electrodes on a share basis to said one input terminal.
Or alternatively, in the distance measuring equipment according to the present invention as mentioned above, it is acceptable that said semiconductor photo detector element comprises said detection surface, signal electrodes, connected to associated divided areas of said detection surface, respectively, adapted for outputting the photo-current generated by irradiation of light onto said detection surface, a second switch, disposed at a boundary between divided areas of said detection surface, for selectively offering a first state which permits the photo-current generated on said detection surface to conduct through the associated boundary, and a second state which inhibits the photo-current generated on said detection surface from conducting through the associated boundary, in accordance with a control signal, and a third switch, provided in association with another signal electrode excepting a first signal electrode which is one of said signal electrodes, for selectively offering a first state which permits the photo-current generated on said detection surface to conduct into the associated signal electrode, and a second state which inhibits the photo-current generated on said detection surface from conducting into the associated signal electrode, in accordance with a control signal, and
said first signal transmission path leads, when said third switch offers said second state, while said second switch offers said first state, the photo-current outputted from the first signal electrode to said one input terminal.
In the distance measuring equipment according to the present invention as mentioned above, said first state is, for example, a low impedance state, and said second state is, for example, a high impedance state. However, those states are not always related to high impedance and low impedance. The first state implies a state in which it is permitted that the photo-current conducts through the switch. The second state implies a state in which it is prohibited that the photo-current conducts through the switch.
In the distance measuring equipment according to the present invention as mentioned above, it is preferable that said semiconductor photo detector element has a plurality of signal electrodes adapted for outputting on a share basis the photo-current generated on said detection surface, and
distance measuring equipment further comprises a transmission path selecting unit for changing over between said first signal transmission path and a second signal transmission path for leading two types of photo-currents outputted from two signal electrodes of the plurality of signal electrodes of said semiconductor photo detector element to the two input terminals of said ratio arithmetic circuit, respectively.
The adoption of the transmission path selecting unit makes it possible to perform a ratio arithmetic processing based on the total of a plurality of photo-currents through changing over to the first signal transmission path, and also possible to perform the conventional ratio arithmetic processing through changing over to the second signal transmission path.
In the distance measuring equipment according to the present invention as mentioned above, it is preferable that said semiconductor photo detector element has a plurality of signal electrodes adapted for outputting on a share basis the photo-current generated on said detection surface, and
distance measuring equipment further comprises a transmission path selecting unit for changing over between said first signal transmission path and a third signal transmission path for leading one photo-current of a plurality of photo-currents outputted from the plurality of signal electrodes of said semiconductor photo detector element to one of the two input terminals of said ratio arithmetic circuit, and for leading a fixed current signal from said current source to another input terminal of the two input terminals of said ratio arithmetic circuit.
In some distance up to the target to be measured or some structure of the semiconductor photo detector element, it happens that an optionally selection of one of the signals representative of photo-currents outputted from the photo detector elements makes it possible to effectively expect an improvement of a performance. In such a case, the transmission path selecting unit is used to change over to the third signal transmission path so that the ratio arithmetic processing with the fixed current signal is performed. This feature makes it possible to enhance an accuracy of focusing in the distance area.
Further, in the distance measuring equipment according to the present invention as mentioned above, it is preferable that said semiconductor photo detector element has a plurality of signal electrodes adapted for outputting on a share basis the photo-current generated on said detection surface, and
distance measuring equipment further comprises a transmission path selecting unit for changing over between said first signal transmission path, a second signal transmission path for leading two types of photo-currents outputted from two signal electrodes of the plurality of signal electrodes of said semiconductor photo detector element to the two input terminals of said ratio arithmetic circuit, respectively, and a third signal transmission path for leading one photo-current of a plurality of photo-currents outputted from the plurality of signal electrodes of said semiconductor photo detector element to one of the two input terminals of said ratio arithmetic circuit, and for leading a fixed current signal from said current source to another input terminal of the two input terminals of said ratio arithmetic circuit.
An adoption of the transmission path selecting unit makes it possible to measure a distance with greater accuracy over the wide distance range. For example, according to the embodiments which will be described later, in the event that the target to be measured in distance is in a sufficiently long distance area, a ratio arithmetic processing based on the light quantity is performed through changing over to the first signal transmission path, in the event that the target to be measured in distance is in a short distance area, the conventional ratio arithmetic processing is performed through changing over to the second signal transmission path, and in the event that the target to be measured in distance is in a slightly long distance area, a ratio arithmetic processing based on the fixed current signal is performed through changing over to the third signal transmission path.
To achieve the above-mentioned object, the present invention provides a camera having a distance measuring equipment for projecting light for focusing in front of the camera and detecting light reflected on a subject to determine a distance up to the subject, said distance measuring equipment moving an image taking lens to the determined distance for photography, wherein said distance measuring equipment comprises:
a semiconductor photo detector element having a detection surface generating a photo-current upon receipt of irradiation of light;
a ratio arithmetic circuit, having two input terminals each for receiving associated one of current signals, for performing a ratio arithmetic operation in accordance with the current signals fed from the two input terminals;
a current source for generating fixed current signal; and
a first signal transmission path wherein the photo-current obtained through irradiation of light onto the detection surface of said semiconductor photo detector is lead to one input terminal of the two input terminals of said ratio arithmetic circuit and the fixed current signal from said current source to another input terminal of the two input terminals of said ratio arithmetic circuit.
In the camera according to the present invention as mentioned above, it is preferable that said semiconductor photo detector element has a plurality of signal electrodes adapted for outputting on a share basis the photo-current generated on said detection surface, and
distance measuring equipment further comprises a transmission path selecting unit for changing over between said first signal transmission path and a second signal transmission path for leading two types of photo-currents outputted from two signal electrodes of the plurality of signal electrodes of said semiconductor photo detector element to the two input terminals of said ratio arithmetic circuit, respectively.
In the camera according to the present invention as mentioned above, it is preferable that said semiconductor photo detector element has a plurality of signal electrodes adapted for outputting on a share basis the photo-current generated on said detection surface, and
distance measuring equipment further comprises a transmission path selecting unit for changing over between said first signal transmission path and a third signal transmission path for leading one photo-current of a plurality of photo-currents outputted from the plurality of signal electrodes of said semiconductor photo detector element to one of the two input terminals of said ratio arithmetic circuit, and for leading a fixed current signal from said current source to another input terminal of the two input terminals of said ratio arithmetic circuit.
Further, in the camera according to the present invention as mentioned above, it is preferable that said semiconductor photo detector element has a plurality of signal electrodes adapted for outputting on a share basis the photo-current generated on said detection surface, and
distance measuring equipment further comprises a transmission path selecting unit for changing over between said first signal transmission path, a second signal transmission path for leading two types of photo-currents outputted from two signal electrodes of the plurality of signal electrodes of said semiconductor photo detector element to the two input terminals of said ratio arithmetic circuit, respectively, and a third signal transmission path for leading one photo-current of a plurality of photo-currents outputted from the plurality of signal electrodes of said semiconductor photo detector element to one of the two input terminals of said ratio arithmetic circuit, and for leading a fixed current signal from said current source to another input terminal of the two input terminals of said ratio arithmetic circuit.
It is noted that cameras provided with various types of distance measuring equipment as mentioned above are included in a camera of the present invention.